Surface-associated sialic acid is an immunological adjuvant.

The influence of neuraminidase on the immunogenicity of heterologous erythrocytes as determined by serum haemagglutination titres was investigated in mice. For this study sheep and rabbit erythrocytes were selected because of their high and low N-acetylneuraminic (sialic) acid content, respectively. Preincubation with neuraminidase resulted in a ten-fold reduction of the immunogenicity of sheep erythrocytes (ShE). By contrast, the immune response to rabbit erythrocytes appeared to be resistant to sialidase treatment. Addition of the extrinsic adjuvant dimethyldioctadecylammonium bromide largely restored the immunogenicity of neuraminidase-treated ShE, but did not change the response to control-treated ShE. The maximal antibody level induced by neuraminidase-treated ShE was lower than that provoked by control ShE. These results suggest that sialic acid is both an intrinsic immunological adjuvant and an antigenic determinant of ShE. The adjuvant effect of sialic acid does not depend on complement component C3 as judged by the response of cobra venom factor-pretreated animals. In genetically C5-deficient and in nude mice, however, sialic acid showed diminished and absent adjuvant activity, respectively.